Including dispersion in configuration interaction-singles calculations forthe spectroscopy of chromophores in solution

In this paper we prove that a configuration interaction electronic structur e calculation on a supermolecule that contains only single excitations incl udes dispersion interactions between the two subsystems when energy differe nces are taken between the Hartree-Fock (molecular orbital) ground state an d low energy excited states in which single excitations dominate. This theo rem is proven up to second order in perturbation theory. (C) 2000 American Institute of Physics. [S0021-9606(00)51816-9].